java - @JsonTypeResolver 是使用多个属性解析的唯一选项吗？

Question

我有以下格式的传入 JSON 数据

{
    "header": {
        "schema_id": {
            "namespace": "omh",
            "name": "physical-activity",
        },
    },
    "body": {
        "activity_name": "walking",
        "distance": {
            "value": 1.5,
            "unit": "mi"
        },
    }
}

和相应的 Java 类看起来像

public class DataPoint<T extends Measure> {

    private DataPointHeader header;
    private T body;

和

@JsonNaming(LowerCaseWithUnderscoresStrategy.class)
public class PhysicalActivity extends Measure {

    private String activityName;
    private LengthUnitValue distance;

我希望 Jackson 根据 JSON 文档中的 schema_id 将 body 解析为 PhysicalActivity 类型，例如在伪代码中

if schema_id.namespace == 'omh' && schema_id.name == 'physical-activity'
     then return PhysicalActivity.class

我已经尝试使用 @JsonTypeIdResolver 执行此操作，但是如果我尝试使用 @JsonTypeInfo 导航到 header.schema_id.name，例如

@JsonTypeInfo(use = JsonTypeInfo.Id.CUSTOM,
        include = JsonTypeInfo.As.EXTERNAL_PROPERTY,
        property = "header.schema_id.name")
@JsonTypeIdResolver(DataPointTypeIdResolver.class)
public abstract class Measure {

我收到一个missing property: 'header.schema_id.name' 错误。即使可以，我也不认为我可以同时对 namespace 和 name 属性做出决定。

除了使用 @JsonTypeResolver 从头开始​​构建之外，还有其他明智的方法吗？

Answer 1

在 Jackson 的源代码中，似乎有很多类型 ID 是字符串的假设，所以我怀疑 JsonTypeResolver 是一种可行的方法……虽然它看起来并不简单!

至少当您只有“header”和“body”属性时，完全自定义的反序列化器并不太难:

public static class DataPointDeserializer extends StdDeserializer<DataPoint<?>> implements ResolvableDeserializer {
    private JsonDeserializer<Object> headerDeserializer;
    private Map<SchemaId, JsonDeserializer<Object>> activityDeserializers;

    public DataPointDeserializer() {
        super(DataPoint.class);
    }

    @Override
    public void resolve(DeserializationContext ctxt) throws JsonMappingException {
        headerDeserializer = ctxt.findRootValueDeserializer(ctxt.getTypeFactory().constructType(
                DataPointHeader.class));
        activityDeserializers = new HashMap<>();
        activityDeserializers.put(new SchemaId("omh", "physical-activity"),
                ctxt.findRootValueDeserializer(ctxt.getTypeFactory().constructType(PhysicalActivity.class)));
    }

    @Override
    public DataPoint<?> deserialize(JsonParser p, DeserializationContext ctxt) throws IOException,
            JsonProcessingException {
        String fieldName = p.nextFieldName();
        if (fieldName == null)
            throw ctxt.wrongTokenException(p, JsonToken.FIELD_NAME, "expected 'header' and 'body' fields");
        if (fieldName.equals("header")) {
            p.nextToken();
            DataPointHeader header = (DataPointHeader) headerDeserializer.deserialize(p, ctxt);
            JsonDeserializer<Object> bodyDeserializer = activityDeserializers.get(header.schemaId);
            if (bodyDeserializer == null) throw ctxt.mappingException("No mapping for schema: " + header.schemaId);
            fieldName = p.nextFieldName();
            if (fieldName == null)
                throw ctxt.wrongTokenException(p, JsonToken.FIELD_NAME, "expected 'body' field after header");
            p.nextToken();
            Measure body = (Measure) bodyDeserializer.deserialize(p, ctxt);
            DataPoint<Measure> dataPoint = new DataPoint<>();
            dataPoint.header = header;
            dataPoint.body = body;
            return dataPoint;
        }
        else if (fieldName.equals("body")) {
            p.nextToken();
            try (TokenBuffer tb = new TokenBuffer(p)) {
                tb.copyCurrentStructure(p);
                fieldName = p.nextFieldName();
                if (fieldName == null)
                    throw ctxt.wrongTokenException(p, JsonToken.FIELD_NAME, "expected 'header' field after body");
                if (!fieldName.equals("header"))
                    throw ctxt.weirdStringException(fieldName, DataPoint.class, "Unexpected field name");
                p.nextToken();
                DataPointHeader header = (DataPointHeader) headerDeserializer.deserialize(p, ctxt);
                JsonDeserializer<Object> bodyDeserializer = activityDeserializers.get(header.schemaId);
                if (bodyDeserializer == null)
                    throw ctxt.mappingException("No mapping for schema: " + header.schemaId);
                JsonParser bodyParser = tb.asParser();
                bodyParser.nextToken();
                Measure body = (Measure) bodyDeserializer.deserialize(bodyParser, ctxt);
                DataPoint<Measure> dataPoint = new DataPoint<>();
                dataPoint.header = header;
                dataPoint.body = body;
                return dataPoint;
            }
        }
        else throw ctxt.weirdStringException(fieldName, DataPoint.class, "Unexpected field name");
    }
}